The objectives of this contract are as follows: 1) determine the prevalence of retrovirus seropositivity in first time blood donors; 2) determine the rate of retrovirus seroconversion in repeat blood donors as a measure of incidence of infection; 3) ascertain risk factors for antibody-positive donors; 4) characterize the blood donor population by geographic location, age, sex, race/ethnicity, and donation history to permit analysis on prevalence, incidence, and risk factors; 5) identify recipients of retrovirus-positive blood units and conduct clinical and laboratory follow-up of these recipients to determine the transmissibility of these agents via transfusion; and 6) establish blood specimen repositories for long-term storage of specimens from study donors and recipients for future testing.